Pick up something, and hold A to jump to your maximum height. When you get to the top of your jump, throw the item with B and immediately jump (hold A) again. The timing is a little tricky, but you'll be on top of it (literally) soon enough.
Drive up walls:
When you're level 4 in driving, grab an SUV and head for any tunnel. When you come up on a vertical wall, drive along it until you're about to flip, then power up and press and hold B You *should* stick to the wall!
Launch your Ride:
While in co-op activate KEYS TO THE CITY , spawn two ramp trucks , make sure they are back to back , spawn another car (buggy, race car , suv etc.) place it on one of the ramp trucks , get inside the car and have the other player back up the other truck (so the ramps cradle the car you are in ) and the car lauches into the air.